In this paper, an electro-thermal model is proposed for the first time to accurately investigate the self-heating effects in gate-all-around (GAA) silicon nanowire MOSFETs (SNWTs) for thermal-aware design optimization. The model is derived from the equivalent thermal network method, in which the impacts of gate length dependence, nanowire diameter dependence and surface roughness on the nanowire channel thermal conductivity as well as the influence of unique GAA structure features on the heat dissipation are taken into account. The proposed model agrees well with the experimental results of SNWTs. Based on the model, the impacts of structure parameters on the current driving capabilities and heat dissipation of SNWTs are discussed. The developed electro-thermal model can be further applied to the thermal-aware design of SNWT-based circuits.